Note: Descriptions are shown in the official language in which they were submitted.
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Refractory plug or brick for iniecting gas into molten metal.
Description.
[0001] The present invention relates to a refractory plug or brick for
injecting gas into
molten metal and to the mantifacture of a refractory plug or brick for
injecting gas into
molten metal.
[0002] Gases are often injected into molten metal in vessels such as ladles,
crucibles
or tundishes for diverse purposes. For instance, a gas may be introduced into
the
bottom part of a vessel to clear the relatively cool bottom area of
solidification
products, e.g. to remove them from the vicinity of a bottom pour outlet where
the
vessel has such an outlet. In steel making for example, the use of slow
injection of a
fine curtain of gas bubbles in the tundish assists in inclusion removal; the
inclusions
being attracted to the fine gas bubbles and rising upwards through the melt to
the
surface where they are conventionally captured by the tundish cover powder or
flux.
Gas may also be introduced for rinsing or to homogenise the melt thermally or
compositionally, or to assist in dispersing alloying additions throughout the
melt.
[0003] Usually, an inert gas is used but reactive gases may also be employed,
e.g.
reducing or oxidising gases, when the melt compositions or components thereof
needs
modifying. For example, it is customary to inject gases such as nitrogen,
chlorine,
freon, sulphur hexafluoride, argon, and the like into molten metal, for
example molten
aluminium or aluminium alloys, in order to remove undesirable constituents
such as
hydrogen gas, non-metallic inclusions and alkali metals. The reactive gases
added to
the molten metal chemically react with the undesired constituents to convert
them into
a form such as a precipitate, a dross or an insoluble gas compound that can be
readily
separated from the remainder of the melt. These gases (or others) might also
be used
for example with steel, copper, iron, magnesium or alloys thereof.
[0004] In order to efficiently carry out a gas injection operation, it is
desirable that the
gas be introduced into the molten metal, preferably from the bottom of the
recipient, in
the form of a very large number of extremely small bubbles. As the size of gas
bubbles
decreases, the number of bubbles per unit volume increases. An increase in the
number of bubbles and their surface area per unit volume increases the
probability of
the injected gas being utilised effectively to perform the expected operation.
[0005] Previous gas injection proposals have included the installation of a
solid porous
refractory plug or brick in the refractory lining of the vessel, generally on
the floor but
also in the walls. In use, the plugs or bricks introduce a flow of gas in the
form of
bubbles.
[0006] For example, a known technique for introducing gas into molten metal
consists
of lining a portion of a molten metal-containing vessel (preferably the floor
of the vessel)
with a porous ceramic body. The gas is introduced into the porous body at a
location
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remote from the metal-contacting surface of the body. During its passage
through the
body, the gas follows a number of small, tortuous paths such that a large
number of
bubbles will be issued into the molten metal.
[0007] Generally a metal casing that acts as a manifold to introduce gas into
the body
supports the porous ceramic body. Typically, the casing is made of mild steel
(for use
with inert or slightly reactive gas such as argon or nitrogen) or inconel (for
use with
highly reactive chlorine or freon). The assembled body/casing is surrounded
and
supported on all sides except its upper surface by refractory material such as
low-
cement alumina castable or bricks. When castable is used this can either be
cast is
situ around the porous body or formed from pre-cast components fixed in place
during
the hot metal container lining installation. The lining material will "butt"
up against
the porous body construction.
[0008] A problem with the foregoing construction is that it is difficult to
maintain an
effective gas seal between the casing and the body, and between the casing and
the
support castable/bricks. One difficulty arises in part because the
coefficients of
thermal expansion of the metal casing and the refractory materials are
considerably
different; also, the metal casing is subject to attack if chlorine is the gas
being used. If
a crack should develop (as used herein, the term "crack" refers to any defect
in the gas
dispersing apparatus that permits undesired gas leakage), gas will leak
through the
crack, and hereafter frequently will migrate through the next brick and the
refractory
support to the ambient atmosphere. Gas migration through 50 cm or more or
refractory material is possible. The problem is undesirable as the effect of
gas leakage
upon the flow of gas through the designed gas bubbling surface can be
seriously
reduced and effectiveness of the bubbling block diminished. In some cases gas
flow by
means of fine gas bubbles will cease and be replaced with uncontrolled
direction of gas
flow by means of large ineffective gas bubbles. If argon is being used the
relatively
great expense must be considered. The problem is particularly acute in the
case of
chlorine due to the harmful effects of chlorine upon release into the
atmosphere.
Regardless of the type of purifying gas being used, it is important that
cracks be
prevented so that gas leakage will be prevented.
[0009] Desirably, a technique would be avaiiable for injecting gas into molten
metal
that would accomplish the objectives of dispersing a large number of
exceedingly small
bubbles into the molten metal while, at the same time, avoiding cracks in the
gas
dispersing apparatus that results in gas leakage.
[0010] It also would be desirable for any such apparatus to be capable of
being
manufactured easily, at reasonable expense and having smaller dimensions than
the
existing apparatus. Further, it would be desirable that any such gas injection
apparatus be usable with existing equipment such as a tundish, ladle, melting
vessel,
and the like, with no modification, or with only minor modification, of the
existing
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equipment.
[0011] Further, in order to insert this apparatus into the existing refractory
lining of
the molten metal vessel, it would be desirable that any such gas injection
apparatus be
compatible with the surrounding refractory materials to prevent any adverse
chemical
reactions of thermal expansion miss-matches.
[0012] Further, it would be desirable to provide an apparatus that could be
adjusted to
a very broad range of bubbling conditions (bubble size, volume, pressure,
etc.) by only
minor adjustments during the manufacturing process so that the apparatus can
suit
specific customer requirements.
[0013] The invention relates thus to a solid porous refractory plug or brick
for injecting
gas into molten metal through a molten metal-contacting surface comprising
i) a porous refractory body substantially surrounded by a substantially non-
porous
body except at the molten-metal contact surface; and
ii) gas conducting means for conveying the gas from a gas source to the porous
body.
[0014] In the scope of the present specification a plug or brick for injecting
gas can be
a plug, brick, block, dam, tile, bar, and the like. As discussed above, the
plug or brick
of the invention can be used to inject any gas (whether reactive or inert)
into any
molten metal or alloy thereof. The plug or brick has at least a molten-metal
contacting
surface through which the gas is injected. The plug or brick comprises a
porous
refractory body substantially surrounded by (for example encased by or
embedded in) a
non-porous body except, of course, at the molten-metal contacting surface. It
can be
included or form part of the lining of the molten metal vessel.
[0015] The porous body can be made of any porous refractory material. As a
matter of
fact, the nature of the material used is not essential as far as said material
has the
required porosity. Generally, it will be considered that a material having an
apparent
porosity higher than 20 % is porous. Suitable materials typically comprise
alumina,
alumina spinel, magnesia or magnesia spinel, or combinations of any of the
above.
[0016] The plug or brick also comprises gas conducting means for conveying the
gas
from a gas source, to the porous body. The gas conducting means generally
comprises
a conduit extending through the side wall of the non-porous body. This conduit
can be
made from metal or a refractory material for example. The conduit can be fixed
into
place by means of a conventional refractory sealing material (mortar or
cement) or it
can be pressed within the non-porous body.
[0017] Conventional gas conducting means can be utilised. However, since the
tightness of the connection is to be particularly monitored, special
arrangements such
as for example these disclosed in WO-A1-01/83138 are particularly preferred.
[0018] It is also advantageous that the gas conducting means comprises a
plenum
chamber through which the gas contacts a surface of the porous body at least
substantially equivalent to the molten metal contacting surface so that the
gas is
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perfectly homogeneously distributed into the porous body and, consequently,
will
bubble into the molten metal through substantially the whole molten-metal
contacting
surface.
[0019] This type of plug or brick of injecting gas into molten metal is known
for
example from USP 5,054,749, 5,423,521 or 5,219,514. However, none of them
satisfies the above identified requirements.
[0020] The plug or brick of the present invention is characterised by the fact
that the
non-porous body is made of refractory material and that the porous and non-
porous
bodies have been co-pressed. All the above identified requirements are
fulfilled with
such a plug or brick.
[0021] Again, the nature of the non-porous material is not essential as long
as it is a
refractory material and has the required porosity. Generally, it will be
considered that
a material having an apparent porosity lower than 20 % is non-porous.
[0022] The non-porous body and the porous body are preferably constituted of
refractory materials with similar coefficients of thermal expansion. This
serves to
prevent the formation of cracks upon thermal cycling.
[0023] By use of the present invention, the granulometry and permeability of
the inner
porous body can be carefully and consistently controlled to provide an even
fine pore
structure so that small evenly distributed gas bubbles flow from the molten
metal
contacting surface of the body. This permeability can be readily adjusted via
formulation granulometry changes and a plug or brick according to the
invention can
be manufactured to suit specific individual customer requirements.
[0024] This process route is further advantageous in that high magnesia
content
refractories can be used for the formulations such as magnesia spinel. Such
formulations are more compatible with the composition of steel plant tundihs
linings
which are usually basic (magnesia) based. Chemical and thermal characteristics
are
therefore very similar. Advantageously the porous and non-porous refractory
bodies
have thus a high magnesia content, more than 50 %, preferably more than 80 %
and
even more preferably more than 90 % by weight of the composition.
[0025] Accordingly, similar materials but with different granulometries can be
used for
the porous and non-porous bodies. Thus, it is possible to manufacture a plug
or brick
according to the invention from high magnesia content materials having
different
granulometries.
[0026] By virtue of the co-pressing of the two refractory materials, the
natural low
permeability of the non-porous body prevent gas leakage without having to
resort to
other gas leakage restriction techniques. Another advantage of the co-pressing
is that
a plug or brick for injecting gas with lower overall dimensions, achieving the
required
degree of gas bubbling can be used. This assists in handling of these plug or
brick
during transport and installation in the vessel; in particular in the lining.
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[0027] The co-pressing concept is not restricted to oblong, square, round or
oval
shapes but can also be used to produce any refractory cross section suitable
for co-
pressing. For example a ring co-pressed component can be envisaged which would
be
located to surround the exit ports of a tundish thereby forming a surrounding
stream
5 of gentle rising bubbles though which the hot liquid metal would have to
pass prior to
entry into the continuous casting moulds.
[0028] According to another of its aspect, the invention concerns a process
for the
manufacture of a plug or brick for injecting gas into molten metal. According
to the
invention, this process comprises the steps of:
1) introducing into a mould of appropriate amounts of the refractory materials
constituting the porous and non-porous bodies while respecting the desired
limits for these bodies;
2) simultaneous co-pressing of both refractory materials;
3) providing gas conducting means;
4) heat treating the co-pressed materials.
[0029] Preferably, a delimiter, for example made from a thin (but rigid)
plastic or metal
foil is placed into the mould prior to introducing the refractory material.
The delimiter
can be shaped as a cylinder (with circular or oval base), or a parallelepiped,
without
upper and lower surfaces. The refractory material which will form the porous
body is
then introduced in the central portion formed by the delimiter and the
refractory
material which will form the non-porous body is introduced between the
delimiter and
the mould wall. The delimiter is then carefully removed and another amount of
the
material forming the non-porous body is introduced into the mould to form the
surface
opposite to the molten metal contacting surface.
[0030] The step of providing the gas conducting means can be carried out
before or
after the co-pressing step or both before and after. In a preferred embodiment
of the
invention, a plenum chamber is formed by introducing a strip of consumable
material
into the mould at the junction between the base of the porous body forming
material
and the adjacent surface of the non-porous body forming material.
Alternatively or in addition, a hole can be drilled or a conduit placed
through the non-
porous body after or before co-pressing the materials to connect the porous
body
(whether or not through the plenum chamber) to an external gas source.
[0031] The co-pressing step can be carried out according to any known pressing
method, for example in an hydraulic press.
[0032] The heat treating step should be carried out at a temperature
sufficient to
develop a ceramic bond between the porous and non-porous bodies so that the
integrity of the plug or brick and its gas tightness are enhanced. The
consumable
material (if used) placed to generate the plenum chamber will advantageously
be
removed during the heat treating step. This consumable material can burn
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(cardboard, paper) or melt (wax, alloy) at the temperature used. Typically,
the heat
treating step consists in firing the co-pressed material for a temperature
comprised
between 800 and 1800 C for 2 to 12 hours.
[00331 The invention will now be better described with reference to the
enclosed
drawings which are only provided for the purpose of illustrating the invention
and not
to limit its scope. Fig. 1 and 2 show cross-sectional views of embodiments of
the
invention.
[00341 Both figures show a plug or brick (1) for injecting gas into molten
metal through
a molten metal-contacting surface (11) comprising a porous refractory body (2)
substantially surrounded by a substantially non-porous body (9) except at the
molten
metal-contacting surface (11). Also visible on figures 1 and 2 are the gas
conducting
means comprised of a mctallic or refractory conduit (4) extending through a
wall (6) of
the plug or brick and connecting to the plenum chamber (3). The conduit (4) is
typically fixed into place by means of a conventional sealing cement or mortar
(5).
[0035] Advantageously, a gradual tapered section (7) is created towards the
molten
metal-contacting surface (11) during the pressing step as depicted on figure
1. This
taper effect is created during the pressing action by the porous body
deforming into the
non porous medium at the vertical sides of the pressing mould. This tapered
shape
further protects the porous body (2) by forming a key from major spalling
effect.
[0036) According to an example of the invention, the materials used are the
following:
(% given by weight):
Non-porous body Porous body
Silica 0.1 0.13
Alumina 3.3 < 45gm) 0.06
Iron oxide 0.2 0.48
Lime 0.4 0.69
Ma esia 96.0* 98.5**
Granulometry: *: > 1mm 30% **: > 1mm 0%
<45 m 30% <45 m 5%
[0037] After being introduced into the mould, the materials have been
mechanically
pressed at a rate to ensure the best possible compaction and integration of
the co
pressed materials . The heat treating step was carried out by slowly heating
the co-
pressed material at a rate to avoid thermal fractures/cracking within the
pressed body
unti11600 C, leaving the plug or brick at this temperature for 4 hours and
allowing it
to cool gently.
The following properties have been measured:
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41
0
bn
d W ~ U U IJ Q zi
Non-porous body 15.4 2.99 7.11 90.15 6.861
Porous body 24.9 2.59 7.12 52.15 44.762
[0038] In use the plug or brick has injected fine bubbles reliably and
constantly.
